Mr Nunberg is the latest figure to become embroiled in the Russia inquiry.

"Certainly if Mr Nunberg has light to shed about what the president knew about the Trump Tower meeting, we would like to find out," Mr Schiff said, referring to his House Intelligence Committee's investigation on Monday night.

US President Donald Trump has denied any knowledge of that meeting, which has become a central focus of a separate inquiry by special counsel Robert Mueller into alleged Russian meddling in the 2016 election and whether the Trump campaign colluded with the Kremlin.

Congressman Mike Conway, who serves as leading chairman of the House Intelligence Committee, dismissed the idea of calling Mr Nunberg to testify, saying of the Trump Tower meeting: "I think we've pretty much explored that to death."

Mr Nunberg told the New York Daily News on Tuesday he is contemplating seeking treatment for alcohol abuse, a day after his extraordinary series of interviews on live television.

He complained to US media on Monday about being asked to share his email conversations with a long list of ex-campaign aides in response to a subpoena from Mr Mueller's team.

The political operative, who helped launch Donald Trump's presidential campaign before losing his job in 2015, said in a round of media interviews at first he would not comply with Mr Mueller's demand to testify before a grand jury.

But he later told the Associated Press he would probably end up complying.

"I think it would be really, really funny if they wanted to arrest me because I don't want to spend 80 hours going over emails," he told MSNBC.

While he thought investigators believed they had something on Mr Trump, he argued that the subpoena was unfair and added he would like Robert Mueller's team to narrow its scope of inquiry.

Image copyrightGetty ImagesImage caption
Special Counsel Robert Mueller is leading the investigation that hangs over the Trump presidency

White House spokeswoman Sarah Sanders would not be drawn on Mr Nunberg's remarks, saying: "I'm not going to weigh in on somebody that doesn't work at the White House."

Who is Nunberg?

Sam Nunberg worked on the Trump campaign in 2015 until he was fired in August that year over racially charged Facebook posts.

He was later sued by Mr Trump for $10m (£7.2m) for breach of confidentiality.

The lawsuit was "amicably settled" out of court, a lawyer for the Trump Organization said at the time.

'He knew' about Trump Tower meeting

Mr Nunberg said Mr Trump was aware at the time of a June 2016 meeting at Trump Tower when a group of Russians offered his campaign staff damaging information about Hillary Clinton.

"You know he knew about it," Mr Nunberg told CNN. "He was talking about it a week before. I don't know why he went around trying to hide it."

The White House has repeatedly denied Mr Trump knew anything about that meeting.

'Arrest me'

During Monday's interviews with US media, Mr Nunberg said he had met Mr Mueller's team for five-and-a-half hours over the weekend.